Acetaminophen (APAP) toxicity is the leading cause of acute liver failure. The mouse model of APAP hepatotoxicity is highly reproducible and widely utilized to elucidate the mechanisms of liver injury, due to drugs and toxins. Recently, a major role for signal transduction pathways in modulating toxicity downstream of APAP metabolism has been identified using this model. The activation of c-jun-N-terminal kinase (JNK) and its effects on mitochondria have been found to be critical in the signaling pathway leading to APAP-induced necrosis. The goal of this proposal is to elucidate the mechanism of APAP-induced activation of MAPK kinases and the significance, targets, and consequences of the targeting of activated JNK to mitochondria in carrying out the lethal effects of APAP in hepatocytes. The aims are as follows: (1) Determine the role and significance of binding of P-JNK to specific mitochondrial target protein which has been identified in the pathogenesis of APAP hepatotoxicity: Binding partners will be identified by immunoprecipitation and proteomic approaches and the importance of the mitochondrial JNK target will be determined by adenovirus-shRNA knockdown; the effects of pure activated JNK on mitochondrial function and integrity will be assessed: (2) Determine the signal transduction pathways that lead to both early and sustained activation of JNK in APAP toxicity: the interplay of various upstream pathways such as GSK32, Ras/Raf-1, MLK-3, MEKK-1, PKC1, Akt 1+2, will be examined using antisense oligonucleotide silencing and/or knockout mice to elucidate the initial ASK-1 independent activation of JNK required for subsequent ASK-1 dependent sustained JNK activation; (3) Determine the role of ER stress in APAP toxicity: the plausible contribution of ER stress to the signal transduction pathways leading to mitochondrial collapse will be assessed by exploiting knockout or over expression of the key ER stress sensor, GRP78; (4) Determine the role of RIP kinases in APAP-induced necrosis and the relationship to the GSK/JNK/Sab pathway: RIP may play a key role in signal transduction pathways leading to APAP necrosis which will be assessed using specific inhibitors and antisense silencing of RIP-1. The elucidation of these four aims will lead to an improved understanding of the role and interplay of signal transduction pathways and mitochondria in hepatotoxicity and which will identify new therapeutic targets.